In the field of television, considerable efforts have been directed toward developing a digital television receiver wherein the signals are processed by digital circuitry. A motivation for these efforts comes from the fact that the digital television can offer a number of novel features--such as still picture display, multipicture display, direct hookup to a satellite dish amplifier, etc.
There is a continuing desire in the television industry to improve the signal-to-noise ratio (S/N ratio) of the video signals. There are techniques for improving the S/N ratio which take advantage of the fact that video signals are periodic, whereas the noise is aperiodic. Successive addition or signal averaging of the video signals tends to reinforce the periodic signal components, but it tends to cancel out the aperiodic noise components, whereby the S/N ratio is improved. The signal averaging techniques perform well when the television picture is relatively stationary. However, when significant motion is present in the scene, signal averaging causes degradation of the reproduced image--e.g., comet tails.
U.S. patent application, Ser. No. 702,612, filed in behalf of Fling, and entitled "VIDEO SIGNAL RECURSIVE FILTER ADAPTIVELY CONTROLLED RESPONSIVE TO THE HISTORY OF IMAGE MOTION", discloses one technique for dealing with the problem of image motion. The Fling apparatus includes a recursive filter wherein current and stored video signals (which are delayed by one frame period) are proportioned and summed as a function of interimage motion to generate noise reduced video signals. The recursive filter includes a motion detector and an auxiliary frame memory for storing the motion signal at the output of the motion detector. A logic device, responsive to the current and stored motion signal, selects predetermined scale factors (e.g., 1, 1/2 and 1/8) for proportioning the current and delayed video signals.
Typically, the motion detector includes a subtracter for subtracting the stored video signal from the incoming video signal to generate a difference signal. The difference signal, which represents frame-to-frame image motion, is applied to a threshold detector for developing a 1-bit motion signal. The motion signal is a logical zero and a logical one depending upon whether or not the difference signal is less than a threshold level. As previously indicated, the motion signal determines the scale factors used for proportioning the current and delayed video signals in noise reduction systems.
A problem with a motion detector without hysteresis is that when the average difference signal is at or near the threshold level, unwanted signal excursions--such as those produced by noise--cause the detector to switch between the output states, thereby resulting in a spurious motion signal.